Bicarbonate enters erythrocyte from blood plasma.

FIGURE 11-33 Chloride-bicarbonate exchanger of the erythrocyte membrane. This cotransport system allows the entry and exit of HCO;T without changes in the transmembrane electrical potential. Its role is to increase the CO2-carrying capacity of the blood.

two solutes across a membrane. When, as in this case, the two substrates move in opposite directions, the process is antiport. In symport, two substrates are moved simultaneously in the same direction. As we noted earlier, transporters that carry only one substrate, such as the erythrocyte glucose transporter, are uniport systems (Fig. 11-34).

The human genome has genes for three closely related chloride-bicarbonate exchangers, all with the same predicted transmembrane topology. Erythrocytes contain the AE1 transporter, AE2 is prominent in liver, and AE3 is present in plasma membranes of the brain, heart, and retina. Similar anion exchangers are also found in plants and microorganisms.

Active Transport Results in Solute Movement against a Concentration or Electrochemical Gradient

In passive transport, the transported species always moves down its electrochemical gradient and is not accumulated above the equilibrium concentration. Active transport, by contrast, results in the accumulation of a solute above the equilibrium point. Active transport is thermodynamically unfavorable (endergonic) and takes place only when coupled (directly or indirectly) to an exergonic process such as the absorption of sunlight, an oxidation reaction, the breakdown of ATP, or the concomitant flow of some other chemical species down its electrochemical gradient. In primary active transport, solute accumulation is coupled directly to an ex-ergonic chemical reaction, such as conversion of ATP to ADP + Pi (Fig. 11-35). Secondary active transport occurs when endergonic (uphill) transport of one solute is coupled to the exergonic (downhill) flow of a different solute that was originally pumped uphill by primary active transport.

The amount of energy needed for the transport of a solute against a gradient can be calculated from the initial concentration gradient. The general equation for the free-energy change in the chemical process that converts S to P is aG = aG'° + RT ln [P]/[S] (11-1)

where R is the gas constant, 8.315 J/mol • K, and T is the absolute temperature. When the "reaction" is simply

FIGURE 11-34 Three general classes of transport systems. Transporters differ in the number of solutes (substrates) transported and the direction in which each is transported. Examples of all three types of transporters are discussed in the text. Note that this classification tells us nothing about whether these are energy-requiring (active transport) or energy-independent (passive transport) processes.

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